[LocalMatrix] fix iteration when cols == 0
When the local matrix has 0 columns, the iterators where not set up correctly. This merge request fixes this issue by returning the begin iterator as end iterator in such a case.
Edited by Andreas Nüßing
When the local matrix has 0 columns, the iterators where not set up correctly. This merge request fixes this issue by returning the begin iterator as end iterator in such a case.